Download the custom recovery into the same folder as your fastboot file. (I like to rename mine just to keep things short.)
Reboot your phone into the bootloader, either by turning it off and holding vol-up/vol-down+power, or reboot from the command line (adb reboot bootloader).
Plug your phone into your computer if it's not already. (Type fastboot devices to make sure your computer sees it)
In the command line, type fastboot boot xxxxxxx.img (where xxxxxxx is the name of the custom recovery you saved).
And that's it. You'll have booted into the custom recovery, and from there you can do a full backup, or restore a backup, or wipe the phone. All without rewriting anything. If you need a little more hand-holding, there's some video after the break.